Physics only tells you if you can predict the later output from the earlier output. It can’t tell you what you know about the later output. If I flip a coin, it’s random what it will land on before I flip it, but it has a probability near one of having landed on whatever I saw after I flipped it.
Bayesianism is agnostic on the subject—it’s physics that has an opinion. A rather strong opinion.
Physics only tells you if you can predict the later output from the earlier output. It can’t tell you what you know about the later output. If I flip a coin, it’s random what it will land on before I flip it, but it has a probability near one of having landed on whatever I saw after I flipped it.